The sequences of two variant specific proteins (VSPs) have been determined. They reveal that VSPs, even from unrelated isolates, are compositionally and structurally related. Although identity among VSPs are about 50 percent they are antigenically distinct. Inexpressible epitopes are not represented in the genome. VSPs have a common 3' terminal region. If host's respond or can be made to respond to this area, vaccination may be possible. The genomic organization of VSP1267 revealed two identical genes arranged tail to tail with an intervening 3kb region. The VSP repetitive epitope recognized by Mab 6E7 in VSPWB170 is present in other isolates, but the sequences of some are different. The analogue of the G-protein ADP-ribosylation factor has been identified and sequenced in WB and GS Giardia isolates. Although there is near identity at the amino acid level, the sequences are only 87 percent identical indicating significant drift.